10 “Why do You turn away Your hand, and Your right hand from the midst of Your bosom unto the end?”. Again, another sign which was given to Moses. For in like manner as above from the rod was a sign, so also from the right hand now. For when that thing had been done concerning the rod, God gave a second sign: “thrust,” He says, “your hand into your bosom, and he thrust it: draw it forth, and he drew it forth: and it was found white,” that is, unclean. For whiteness on the skin is leprosy, not fairness of complexion.
For the heritage of God itself, that is, His people, being cast out became unclean. But what says He to him? Draw it back into your bosom. He drew it back, and it was restored to its own colour. When doest Thou this, says this Asaph? How long dost Thou alienate Your right hand from Your bosom, so that being without unclean it remains? Draw it back, let it return to its colour, let it acknowledge the Saviour. “Wherefore do you turn away Your hand, and Your right hand from the midst of Your bosom unto the end?”
These words he cries, being blind, not understanding, and God does what He does. For wherefore came Christ? “Blindness in part happened unto Israel, in order that the fullness of the Gentiles might enter in, and so all Israel might be saved.” Therefore now, O Asaph, acknowledge that which has gone before, in order that you may at least follow, if you were not able to go before. For not in vain came Christ, or in vain was Christ slain, or in vain did the grain fall into the ground; but it fell that it might rise manifold. A serpent was lifted up in the desert, in order that it might cure of the poison him that was smitten. Observe what was done. Do not think it to be a vain thing that He came: lest He find you evil, when He shall have come a second time.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)